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A software solution to this is non-trivial. The iMac works as display by by using Target Display Mode. This hides the iMac's video output, while keeping the system running, and the signal you put into the iMac's input gets passed directly through to the iMac's screen.

To do what you want to do, you'll need an an external capture card. The best way to capture natively on your Mac would be something like the Blackmagic Intensity:

The main model is a PCI Express version, so you need a Mac Pro or Hackintosh to use that on OS X. There's a Thunderbolt version if you can use that, and the USB version is 3.0 only, so it won't work on any Mac.

An alternative is something like the Hauppauge HD PVR which can use USB 2.0: